MTU Workarounds

The maximum transmission unit (MTU) is the largest size frame or packet — in bytes or octets (eight-bit bytes) — that can be transmitted across a data link. It is most used in reference to packet size on an Ethernet network using the Internet Protocol (IP). A deprecated term is “window size”. Default is 1500 which is too big for the remote station network. Symptoms are the radio shows up in the Smartlink window but a connection attempt times out.

At least two workarounds are possible which will have no effect on any other applications or users on the router. Which one you use is up to you. Either one works equally well. The first workaround is to change the settings in the main router for the home. Find the settings for MTU in the network configuration and change the MTU to 1438.

The second workaround uses the command line in the pc to do network shell routines. This routine can change the MTU on the PC. Open cmd with administrator permissions (run as administrator by right-clicking on cmd and clicking on “run as administrator”).

Enter these commands:




>show int

Look for the line that shows the connection to the Internet and write down it’s name. An example is “Ethernet”. If you’re using wifi, it might say “Wi-Fi”. Observe the value in the MTU column. Is it 1500?

Enter the following command:

>set subinterface “Ethernet” mtu=1438 store=persistent

where “Ethernet” is the name obtained in one of the steps above.

An “ok” will be returned if these steps worked. To test, enter the following command:

>show int

The results returned will show the current MTU which should now be 1438.

Note that the magic number may be lower or higher than 1438. Experiment if 1438 doesn’t work.

To return to normal, close the cmd window.


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